Neurobiology 104 – 2003

Dr Geoffrey T. Meyer

The Central Nervous System

II. The Central Nervous System (CNS)

The CNS consists of the brain (and retina) and the spinal cord. As in the PNS, CNS tissue consists of two cell types: nerve cells (neurons), and glial cells, which support and protect the neurons.

A. The Spinal Cord.

The spinal cord is continuous with the brain and is divided into a number of segments. Each segment is connected to a pair of spinal nerves and each spinal nerve is joined to its segment of the cord by a number of roots, which are grouped either as posterior (dorsal) or anterior (ventral) roots. The dorsal roots contain afferent or sensory nerves, while the ventral roots contain efferent or motor nerves.

In cross section, the spinal cord exhibits a butterfly-shaped, gray inner substance, the gray matter surrounding the central canal, and a whitish peripheral substance, the white matter. The gray matter contains neuronal cell bodies and their dendrites, as well as axons and glial cells. It is divided into two dorsal and two ventral horns which are connected through the gray commisure. Functionally related groups of nerve cell bodies in the gray matter are called nuclei. Synapses occur only in the gray matter. The white matter contains only myelinated and unmyelinated axons traveling to and from other parts of the spinal cord and the brain, and axons traveling to and from the periphery.

The ventral horn contains the large cell bodies of motor neurons which innervate striated muscle--these are called ventral horn motor neurons. Their axons leave the spinal cord via the ventral root.

Sensory neurons, whose cell bodies are located outside of the spinal cord in dorsal root ganglia, terminate in the dorsal root of the gray matter, where they synapse with interneurons. These interneurons integrate a variety of incoming sensory information, and ascending and descending information to and from the brain, as well as outgoing motor information.

The relative proportions of gray and white matter vary at different levels of the spinal cord. This is due to the greater number of ascending and descending axons at levels of the spinal cord nearer the brain.

B. Meninges

The CNS is protected by the skull and vertebrae. It is also encased in membranes of connective tissue called the meninges. These are the dura mater, arachnoid and pia mater.

The dura mater is the outermost layer of meninges and consists of dense connective tissue, which is continuous with the periosteum of the skull, but is separated from the periosteum of the vertebrae by the epidural space which contains veins, loose connective tissue and adipose tissue. The dura mater is covered by a simple squamous epithelium.

The arachnoid has two components: a layer of dense connective tissue (arachnoid layer) in contact with the dura mater, and an underlying space called the subarachnoid space which is filled with cerebrospinal fluid (CSF). The subarachnoid space is traversed by thin fibers (trabeculae) which connect the arachnoid layer and the pia mater. This space forms a hydraulic cushion that protects the CNS from trauma. The subarachnoid space communicates with the ventricles of the brain. In some areas the arachnoid perforates the dura mater, forming protrusions that terminate in venous sinuses in the dura mater. These protrusions, which are covered by the endothelial cells of the veins, are called arachnoid villi, and are responsible for the reabsorption of CSF into the venous system.

Pia mater consists of loose connective tissue containing many small blood vessels. Pia mater lines the neural tissue and the perivascular space along which blood vessels enter the CNS. Between the pia and the neural elements of the CNS is a thin layer of glial cell processes which form a physical barrier that separates the CNS from the cerebrospinal fluid.

C. Glial cells of the CNS.

Glial cells (neuroglia) make up the major group of cells found in the CNS and outnumber neurons by a ratio of 10 to 1. Glial cells may have several functions, including support, nutrition, repair of damaged tissue, defense, isolation of individual neurons and phagocytosis.

Glial cells include astrocytes, oligodendrocytes, microglia and ependymal cells. Other specialized glial cells may be found in some parts of the CNS.

There are two types of astrocytes, protoplasmic and fibrous astrocytes, which are found in gray and white matter respectively. Astrocytes bind neurons to capillaries and to the pia mater (see below), where they form a continuous layer between the pia and the neurons. One type of astrocyte develops processes with expanded end-feet that are linked to endothelial cells by junctional complexes, and form a continuous barrier between the CNS and the blood vessels. This is one component of the blood-brain barrier. Additionally, they participate in controlling the ionic and chemical environment of neurons. They display many specific receptors on their surfaces, and secrete metabolic substances and molecules which either stimulate or inhibit the metabolism of neurons. When the CNS is damaged, astrocytes proliferate to form scar tissue.

Oligodendrocytes are located in both gray and white matter. They produce the myelin sheath that provides the electrical insulation of axons in the CNS (i.e.. they perform the same function in the CNS as Schwann cells do in the PNS). However, one oligodendrocyte surrounds and myelinates the axons of several neurons. Myelin basic protein and phospholipids are the major components of myelin.

Microglia are small, elongated cells with short processes which are found in small numbers in both gray and white matter. They are phagocytic cells derived from mononuclear cells of the blood. They are also involved in inflammation and repair in the CNS. In multiple sclerosis microglia phagocytose and degrade myelin surrounding the axons of the CNS neurons.

Ependymal cells are low columnar ciliated cells that line the fluid filled cavities of the CNS. They have the morphologic and physiologic characteristics of fluid transporting cells. In several locations in the brain (the choroid plexuses), they are modified to produce cerebrospinal fluid (CSF).

D. Choroid Plexus

The choroid plexus consists of invaginated folds of pia mater that penetrate the interior of the ventricles. It is composed of loose connective tissue of the pia mater, covered by a simple secretory cuboidal epithelium which is continuous with the ependymal layer. The connective tissue contains numerous fenestrated capillaries. The main function of the choroid plexus is to secrete CSF, that completely fills the ventricles, central canal of the spinal cord, subarachnoid space and perivascular space. CSF is important for the metabolism of the CNS and acts as a protective device. It is reabsorbed into the venous sinuses after passing through the arachnoid villi.

E. Ventricular System and Ependymal Cells

The ventricular system of the CNS includes the ventricles of the brain and the central canal. This system is filled with CSF. The cells which line these spaces are known as ependymal cells. In some regions, these cells are specialized to produce CSF. The modified ependymal cells and associated capillaries are called the choroid plexus.

F. The Brain.

The brain is made up of numerous distinct regions, including the cerebral cortex, cerebellum, thalamus, hypothalamus and brainstem. It has virtually no connective tissue, and is therefore a relatively soft, gel-like organ.

An important histological difference between the brain and spinal cord is the distribution of gray and white matter. In brain, gray matter surrounds white matter (in contrast to spinal cord, where white matter surrounds gray matter). The brain is a complex structure containing laminated (layered) as well as non-laminated structures. For illustration we will consider two laminated structures, the cerebral cortex and the cerebellum.

a) The cerebral cortex is a highly folded structure with different laminated regions subserving different roles. Neurons of some regions of the cortex register afferent (sensory) impulses; in other regions, efferent neurons generate motor impulses that control voluntary movements. It plays a wide variety of functional roles, including the final integration of sensory (vision, auditory, smell etc.) information, learning, memory etc. and the initiation of voluntary motor responses. There are many important cell types in the cerebral cortex. One important and easily recognizable cell is the pyramidal cell which projects from the cortex to other brain regions.

b) The cerebellum is also a folded and laminated structure. It plays a critical role in the control of movements. It consists of three layers, the outer molecular layer, the Purkinje cell layer and the inner granular layer. The Purkinje cells have a large cell body, and an extremely complex dendritic arborization. These dendrites occupy most of the molecular layer, and are the reason for the sparseness of nuclei in that layer.